1. Field of the Invention
The present invention relates to a lean burn internal combustion engine, and in particular, relates to a control of an air-fuel ratio and ignition timing in the lean burn internal combustion engine.
2. Description of the Related Art
A known prior art lean burn combustion engine has an operating area, such as a low load condition, wherein a lean combustible mixture is introduced into the engine to increase the fuel consumption efficiency. In this lean burn engine, a basic fuel injection amount, which is an amount of fuel needed for providing a theoretical air-fuel ratio at a combination of an engine speed and engine load parameter such as intake pressure, is first calculated, and then, to obtain the lean air-fuel mixture, a lean correction factor having a value smaller than 1.0 is multiplied by the basic fuel injection amount. A lean correction factor map is provided, which is constructed by values of a lean correction factor with respect to combinations of an engine speed and an intake pressure. When the engine goes from the lean area to a power area, due to a depression of the acceleration pedal, a fuel enrichment correction is carried out to obtain a desired engine torque.
The prior art lean combustion engine suffers from a drawback in that a shock is generated when the engine leaves the lean zone and enters the power mode. In the lean zone, the lean air-fuel mixture is obtained by multiplying the lean correction factor having a value smaller than 1.0 by the basic fuel injection amount corresponding to the theoretical air-fuel ratio. The lean correction factor is calculated from the intake pressure and engine speed by using a map interpolation technique. The value of the intake pressure is initially increased in accordance with the increase in the degree of the depression of the accelerator pedal, to obtain a desired increase in the engine torque. Once the degree of the opening of the throttle valve reaches a predetermined value, however, the value of the intake pressure remains substantially unchanged, and therefore, a desired increase in torque cannot be obtained. When the accelerator pedal is further depressed, the engine enters the power area, from the low torque state, where a fuel enrichment correction is carried out to abruptly increase the engine torque, which causes a shock to be generated.
To overcome this drawback, a system has been proposed wherein a second lean correction factor map is provided, and a lean correction factor is calculated from a combination of the value of a degree of opening of the throttle valve and the engine speed. See U.S. Pat. application No. 528,565, filed on May 24, 1990. At the degree of opening of the throttle valve where the intake pressure remains unchanged, this second map for the lean correction factor based on the throttle opening is used, and as a result, the engine torque can be increased even if the intake pressure remains unchanged, and therefore, an occurrence of torque shock is prevented when the engine enters the power zone.
Nevertheless, the provision of the second lean correction factor suffers from a drawback in that the ignition timing is far from an optimum ignition timing if the ignition timing is calculated by the ignition timing map based on the intake pressure. When the throttle opening map is used for calculating the lean correction factor, the air-fuel ratio can be varied as the degree of the opening of the throttle valve is increased, even if the intake pressure remains unchanged, but the calculation of the ignition timing based on the intake pressure causes the ignition timing to remain substantially unchanged. As is well known, the change in the air-fuel ratio by the employment of the throttle opening map for calculating the lean correction factor causes the desired ignition timing to be changed, but the employment of the intake pressure map for calculating the ignition timing leaves it unchanged. As a result, the prior art provision of a second lean correction factor map based on the degree of throttle opening suffers from a drawback in that a desired ignition timing cannot be obtained.